[GRASS-dev] dealing with nan

[Yann Chemin]

2008/5/19 Glynn Clements <glynn at gclements.plus.com>:

>
> Yann Chemin wrote:
>
> > I have some data that reads as "nan", while rare, it may happen when
> > compressing/uncompressing GRASS Locations to ship them across
> > Internet.
> >
> > I would like to know how to deal with them in GRASS raster
> > programming, so that they are:
> > 1 - detected
> > 2 - set to null
>
>         if (x != x)
>                G_set_d_null_value(&x, 1);
>

Sounds simple enough.


>
> There is also isnan(), which is in C99, and also specified by POSIX:
>
>        http://www.opengroup.org/onlinepubs/009695399/functions/isnan.html
>
> However, I don't know if it exists on all platforms which we care
> about. MSDN says that MSVCRT has _isnan() (with a leading underscore),
> but it's defined in <float.h> rather than <math.h>.
>

hmm... ok



>
> The (x != x) test should be portable; OTOH, it's the kind of thing
> that compilers often get wrong, particularly when optimising (if you
> ignore NaN, x!=x is always false).
>

OK, this is a pickle... I'll give it a try and see what it does here.


>
> For 7.0, I intend to change G_is_[fd]_null_value() to treat all NaN
> values as null, not just the specific bit patterns which it currently
> uses.
>

This would be good indeed.


>
> --
> Glynn Clements <glynn at gclements.plus.com>
>



-- 
Yann Chemin
International Rice Research Institute
Office: http://www.irri.org/gis
Perso: http://www.freewebs.com/ychemin
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